The present invention generally relates to hybrid molds for use in a molten solder screening process for molding solder bumps and methods for preparing such molds and more particularly, relates to hybrid molds for molding a multiplicity of solder balls which are constructed by a crystalline silicon face plate provided with a multiplicity of cavities formed in a front surface and a rigid backing plate which has substantially the same coefficient of thermal expansion as crystalline silicon for bonding to a back surface of the face plate and methods for preparing such hybrid molds.
In modem semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques of such devices. Conventionally, a flip-chip attachment method has been used in the packaging of IC chips. In the flip-chip attachment method, instead of attaching an IC die to a lead frame in a package, an array of solder balls is formed on the surface of the die. The formation of the solder balls is normally carried out by an evaporation method of lead and tin through a mask for producing the desired solder balls. More recently, the technique of electro-deposition has been used to produce solder balls in flip-chip packaging.
Other solder ball formation techniques that are capable of solder-bumping a variety of substrates have also been proposed. These techniques work fairly well in bumping semiconductor substrates that contain solder structures over a minimum size. One of the more popularly used techniques is a solder paste screening technique which can be used to cover the entire area of an 8 inch wafer. However, with the recent trend in the miniaturization of device dimensions and the reduction in bump-to-bump spacing (or pitch), the solder paste screening technique becomes impractical. For instance, one of the problems in applying solder paste screening technique to modem IC devices is the paste composition itself. A paste is generally composed of a flux and solder alloy particles. The consistency and uniformity of the solder paste composition become more difficult to control with a decreasing solder bump volume. A possible solution for this problem is the utilization of solder pastes that contain extremely small and uniform solder particles. However, this can only be done at a high cost penalty. Another problem in using the solder paste screening technique in modern high density devices is the reduced pitch between bumps. Since there is a large reduction in volume from a screened paste to the resulting solder bump, the screen holes must be significantly larger in diameter than the final bumps. The stringent dimensional control of the bumps makes the solder paste screening technique impractical for applications in high density devices.
A more recently developed injection molded solder (IMS) technique attempted to solve these problems by dispensing molten solder instead of solder paste. However, problems have been observed when the technique is implemented to wafer-sized substrates. U.S. Pat. No. 5,244,143, discloses the injection molded solder technique and is hereby incorporated by reference in its entirety. One of the advantages of the IMS technique is that there is very little volume change between the molten solder and the resulting solder bump. The IMS technique utilizes a two inch wide head that fills boro-silicate glass molds that are wide enough to cover most single chip modules. A narrow wiper provided behind the solder slot passes the filled holes once to remove excess solder. The IMS method for solder bonding is then carried out by applying a molten solder to a substrate in a transfer process. When smaller substrates, i.e., chip scale or single chip modules (SCM""s) are encountered, the transfer step is readily accomplished since the solder-filled mold and substrate are relatively small in area and thus can be easily aligned and joined in a number of configurations. For instance, the process of split-optic alignment is frequently used in joining chips to substrates. The same process may also be used to join a chip-scale IMS mold to a substrate (chip) which will be bumped.
A more recently developed method that does not have the limitations of the solder paste screening technique of significant volume reductions between the initial paste and the final solder volume is the molten solder screening (MSS) method. In the MSS method, pure molten solder is dispensed. When the MSS solder-bumping method is used on large substrates such as 8 inch or 12 inch wafers, surface tension alone is insufficient to maintain intimate contact between a mold and a substrate. In order to facilitate the required abutting contact over large surface areas, a new method and apparatus for maintaining such are therefore necessary.
For instance, in a co-pending application of Ser. No. 09/070,021 commonly assigned to the Assignee of the present application and is hereby incorporated by reference in its entirety, a method for forming solder bumps by a MSS technique that does not have the drawbacks or shortcomings of the conventional solder bumping techniques has been proposed. In the method, a flexible die member is used in combination with a pressure means to enable the die member to intimately engage a mold surface and thus filling the mold cavities and forming the solder bumps. The flexible die head also serves the function of a wiper by using a trailing edge for removing excess molten solder from the surface of the mold.
The MSS process can be carried out by first filling a multiplicity of cavities in the surface of a mold with molten solder. This is accomplished by first providing a stream of molten solder and then passing a multiplicity of cavities in the mold surface in contact with the surface of the stream while adjusting a contact force such that the molten solder exerts a pressure against the surface of the mold to fill the cavities with solder and to remove excess solder from the surface of the mold. The stream of molten solder is supplied through a die head constructed of a flexible metal sheet that is capable of flexing at least 0.0015xe2x80x3 per inch of the die length. The solder has a composition between about 58% tin/42% lead and about 68% tin/32% lead. The multiplicity of cavities each has a depth-to-width aspect ratio of between about 1:1 and about 1:10. The mold body is made of a material that has a coefficient of thermal expansion substantially similar to that of silicon or the final solder receiving material. The contact between the multiplicity of cavities and the surface of the molten solder stream can be adjusted by a pressure means exerted on the flexible die.
Referring now to FIG. 1A, wherein a flexible die head 10 for forming solder bumps by a molten solder screening process is shown. The die head 10 has a die body 12 which is made of a thin, flexible metal sheet such as stainless steel or any other suitable material that is non-wetting to solder. The die body 12 has a gate opening 14 and a slot opening 16. The gate opening 14 has a narrow width and is generally positioned at the center of the die body 12. The gate opening 14 provides fluid communication between a front side 18 and a back side 22 of the die body 12. The gate opening 14 further provides a passageway for receiving a molten solder 24 stored in a solder reservoir 26. The molten solder 24 is kept under an inert gas atmosphere at a pressure of approximately 5 psi. A suitable inert gas used is nitrogen, i.e., as shown in FIG. 1A, supplied by a nitrogen source 28. The functions of the inert gas in the solder reservoir are two fold. First, it provides an inert gas blanket over the solder 24 such that any oxidation of the molten solder 24 can be avoided. Secondly, the nitrogen blanket in the reservoir 26 provides a positive pressure such that molten solder 24 flows easily through the gate opening 14 into the slot opening 16. In continuous operation, the nitrogen pressure is turned off when flexible die head 10 moves between molds to prevent solder loss. The slot opening 16 is provided in the front surface 18 of the die body 12 in a suitable depth such that molten solder 24 can easily flown into the mold cavities 32 (shown in FIG. 2). The width of the slot opening 16 is predetermined such that it covers substantially all the cavities 32, 38 in the mold surface 42. The opening 16 is also wide enough to cover the entire width of a wafer surface to be later bumped by first filling a mold surface having the same width.
The die body 12 also functions as a high temperature squeegee which separates the molten solder in the slot opening 16 from the molten solder filled in the mold cavities 32 (FIG. 2). In order to accomplish this task, the die body 12 must be positioned closely behind a molten solder flow front when the flow front completely covers a linear area across the mold surface 42. The aspect ratio (the depth-to-width ratio) of the mold cavities 32 are typically 0.5 so that solder flows easily into and penetrates to the bottom of the cavity. It has been noted that, at this point, it is critical to xe2x80x9ccutxe2x80x9d or xe2x80x9cseverxe2x80x9d the molten solder as the mold plate 34 scans over the molten solder flow. This difficult task is accomplished by the flexible die body 12 in a unique manner since the trailing edge 36 of the die body 12 functions as a flexible wiper, or squeegee, to continuously scrape the surface 42 of the mold plate 34. FIG. 1B illustrates a bottom view of the flexible die head 10 shown in FIG. 1A. The trailing edge 36 of the die body 12 therefore effectively xe2x80x9ccutsxe2x80x9d the solder supply 24 from the molten solder that has already been deposited in the mold cavities 32. The trailing edge 36, should be sufficiently smooth to assure a uniform contact across the optically-smooth mold surface 42. It is another unique feature that the trailing edge 36, or the wiper, of the die body 12 is flexible only on a global scale, i.e. on a scale of the width of the mold plate 34. As a result, the trailing edge 36 does not enter into cavities 32 and damage the solder bumps formed in the cavities. The word xe2x80x9cflexibilityxe2x80x9d used in the context of the application is on the scale of inches, while the word xe2x80x9crigidityxe2x80x9d used in the context is on the scale of thousandths of an inch or mils.
As shown in FIG. 2, the flexible die body 12 scans smoothly over the surface 42 of the mold plate 34, i.e., over the top of all the cavities 32 allowing the solder within the cavities to stay while removing excess solder from the surface 42. This operation continues as the mold plate 34 is scanned over the molten solder supply 24 until all the cavities are filled. As shown in FIG. 2, the cavities 38 not yet scanned over die body 12 are still empty. The method only requires the die body 12 to pass over the mold plate 34 once for a complete fill. The novel process therefore eliminates solder streaking and non-uniform fill problems caused by multiple scannings with overlapped areas encountered in conventional methods.
The MSS method is therefore a new technique for solder bumping large 8 inch or even 12 inch silicon wafers. As previously described, the technique basically involves filling cavities in wafer-sized mold plates with molten solder, solidifying the solder and then transferring the solder in these cavities to the wafer. The transfer process requires aligning the cavities in a mold plate to the solder receiving pads on a silicon wafer and then heating the assembly to a solder reflow temperature. This results in the molten solder to metallurgically bond to the metallized pads on the wafer and thus assuring the solder in each cavity to transfer from the mold plate to the wafer. Since various solder alloys are readily processed with the MSS technique, the mold plate and wafer assembly must remain aligned throughout the reflow process. Since the contact area between mold plate and wafer covers an entire 8 inch or 12 inch silicon wafer, it is important that these materials match very closely in coefficient of thermal expansion (CTE), for instance, when the mold plate is fabricated of a borosilicate glass.
In another copending application Ser. No. 09/019,396 assigned to the common assignee of the present invention, a process for etching a glass mold plate is disclosed for producing the desired cavities in a mold for receiving molten solder. However, since glass is an amorphous material, processing parameters which control the isotropic etching must be carefully monitored to produce the desired cavity volumes. Even when such control is possible, the resulting cavity has a flat bottom with curved sidewalls which allows the reflowed solder ball certain degree of lateral movement before bonding to the solder receiving pad on a wafer or any other electronic substrates. It is desirable to eliminate any possibility of such lateral movement such that highest accuracy of ball location during the reflow process can be maintained.
It is therefore desirable to provide cavities for solder balls that are not hemispherical in shape such that the location of the solder ball can be controlled more accurately. Since the substrate that typically receives solder bumps is an 8 inch round silicon wafer, the corresponding hole pattern in the mold plate is also circular. It is known that the MSS head has a solder slot which is slightly greater than the diameter of the circular hole pattern, i.e., about 8 inches, thus a run-on and run-off area is required at the beginning and at the end of the scan length. For instance, this is so when the mold is a 10xe2x80x3xc3x9710xe2x80x3 square borosilicate glass plate which has an 8 inch circular hole pattern etched therein. However, when the mold plate is made of an 8 inch anistropically etched  less than 100 greater than  silicon wafer, there is no run-on or run-off area, since the wafer diameter is only slightly larger than the circular hole pattern area. Thus, there is a need to square-off a round silicon wafer mold plate to provide the peripheral area needed by a MSS solder head. Even though it is possible to take a larger 12 inch round  less than 100 greater than  silicon wafer and etch a central 8 inch area to produce the hole pattern to bump an 8 inch silicon wafer, it is undesirable for several reasons. First, since silicon wafers are crystalline material, they are sensitive to defects in the crystal which may initiate and propagate cracks. Secondly, since the MSS process subjects the  less than 100 greater than  silicon mold plate to mechanical stress, standard ratios of wafer diameter-to-thickness would be insufficient to prevent possible fatigue cracking. Furthermore, if the wafer to be bumped was 12 inch in diameter, then an even larger, i.e., 16 inch diameter mold plate wafer would be required. As a consequence, the largest manufactured silicon wafer could not be bumped by a silicon-only mold plate.
It is therefore an object of the present invention to provide a hybrid mold for molding a multiplicity of solder balls that does not have the drawbacks or shortcomings of the conventional molds.
It is another object of the present invention to provide a hybrid mold for molding a multiplicity of solder balls that consists of a crystalline silicon face plate and a backing plate bonded to the face plate.
It is a further object of the present invention to provide hybrid mold for molding a multiplicity of solder balls wherein a crystalline silicon face plate is utilized by etching in its surface along a crystallographic orientation a multiplicity of cavities.
It is another further object of the present invention to provide a hybrid mold for molding a multiplicity of solder balls wherein a multiplicity of cavities are formed in a front surface of a crystalline silicon face plate with each of the cavities being a pyramidal shape.
It is still another object of the present invention to provide a hybrid mold for molding a multiplicity of solder balls wherein a crystalline silicon face plate and a backing plate made of a material having a rigidity and a coefficient of thermal expansion substantially similar to that of the crystalline silicon are used.
It is yet another object of the present invention to provide a hybrid mold for a multiplicity of solder balls by etching a multiplicity of cavities in a crystalline material in an anisotropic etching process thus eliminating the need for the careful monitoring of processing parameters since the crystallographic nature of the crystalline material determines the etching geometrics.
It is still another further object of the present invention to provide a hybrid mold for a multiplicity of solder balls by bonding a crystalline silicon face plate to a borosilicate glass backing plate by an adhesive means.
It is yet another further object of the present invention to provide a hybrid mold for a multiplicity of solder balls by bonding a crystalline silicon face plate to a borosillcate glass backing plate such that the crystalline silicon face plate has a coefficient of thermal expansion substantially similar to that for the electronic substrate onto which the multiplicity of solder balls are transferred.
It is still another further object of the present invention to provide a hybrid mold for a multiplicity of solder balls by bonding a crystalline silicon face plate that has a multiplicity of cavities formed in a front surface to a borosilicate glass backing plate such that the front surface of the face plate and a top surface of the backing plate are coplanar.
In accordance with the present invention, a hybrid mold for a multiplicity of solder balls and a method for preparing such hybrid mold are disclosed.
In a preferred embodiment, a hybrid mold for a multiplicity of solder balls is provided which includes a crystalline silicon face plate that has a multiplicity of cavities formed in a front surface and a backing plate bonded to a back surface of the face plate. The multiplicity of cavities in the front surface of the crystalline silicon face plate is formed along a crystallographic orientation, such as  less than 100 greater than . Each of the multiplicity of cavities may have a pyramidal shape. The mold can be used to transfer solder balls to an electronic substrate in a molten solder screening process.
The backing plate of the hybrid mold may have a rigidity that is substantially similar to that of the crystalline silicon. The backing plate may further have a coefficient of thermal expansion substantially similar to that of the crystalline silicon, i.e., a coefficient of thermal expansion within 50% of that for the crystalline silicon. The backing plate may be formed of a ceramic such as glass. The backing plate may be formed of borosilicate glass. The backing plate of the hybrid mold may further be formed of a polymer which has a rigidity similar to that for the crystalline silicon. The backing plate may be bonded to the face plate by adhesive means, or by a thermal ionic means, such as Mallory(copyright) bonding. The backing plate may also be formed by casting a molten glass around the face plate or by casting a flowable polymer around the face plate. The backing plate may further be bonded to the face plate by a polymeric based adhesive. The crystalline silicon face plate has a coefficient of thermal expansion that is substantially similar to that for the electronic substrate onto which the multiplicity of solder balls are transferred.
The present invention is further directed to a hybrid mold for a multiplicity of solder balls that includes a crystalline silicon face plate which has a substantially parallel front surface and back surface, a multiplicity of cavities for forming solder in the front surface of the face plate, and a backing plate which has a top surface and a bottom surface, the top surface has a recess formed therein for receiving the face plate by adhesive means such that the front surface of the face plate and the top surface of the backing plate are substantially coplanar.
The multiplicity of cavities for forming solder balls in the hybrid mold are formed along a preselected crystallographic orientation, such as an orientation of  less than 100 greater than . Each of the multiplicity of cavities may have a pyramidal shape. The recess in the top surface of the backing plate has an area and a depth sufficiently large for receiving the face plate such that the front surface of the face plate and the top surface of the backing plate are coplanar when the face plate is bonded to the recess by an adhesive layer. The backing plate may be formed of a material that has a rigidity and coefficient of thermal expansion substantially similar to those for crystalline silicon. The backing plate may be formed of borosilicate glass.
The present invention is further directed to a hybrid mold for a multiplicity of solder balls that includes a crystalline silicon face plate which has a first diameter and a multiplicity of cavities formed in a front surface, a backing plate which has a top surface bonded to a back surface of the face plate, the backing plate has a length and a width larger than the first diameter of the face plate, a frame member which has substantially the same length and width as the backing plate and an aperture having a second diameter larger than the first diameter of the face plate, and an adhesive circumferentially bonding the frame member to the face plate such that a top surface of the frame member is coplanar with the front surface of the face plate.
The hybrid mold may further include an adhesive layer between a bottom surface of the frame member and a top surface of the backing plate. The frame member and the backing plate may be formed substantially of the same material. The frame member and the backing plate may be formed of a material that has a rigidity substantially similar to that of crystalline silicon. The frame member and the backing plate may be formed of a material which has a coefficient of thermal expansion within 50% of that for the crystalline silicon. A thermal ionic bond may exist between the top surface of the backing plate and the back surface of the face plate. The multiplicity of cavities are formed in the crystalline silicon along a crystallographic orientation of  less than 100  greater than . Each of the multiplicity of cavities in the hybrid mold may be formed in a pyramidal shape.
The present invention is still further directed to a hybrid mold for a multiplicity of solder balls which consists of a crystalline silicon face plate having a multiplicity of cavities formed in a front surface, and a backing plate encasing the face plate with the front surface of the face plate exposed and a top surface of the backing plate being substantially coplanar with the front surface of the face plate.
The backing plate of the hybrid mold may be formed of a material that has a rigidity and a coefficient of thermal expansion substantially similar to those of the crystalline silicon. The multiplicity of cavities may be formed in the crystalline silicon along a crystallographic orientation of  less than 100 greater than . The backing plate of the hybrid mold may be formed of borosilicate glass or of a polymeric material.
The present invention is still further directed to a method for preparing a hybrid mold for a multiplicity of solder balls which can be carried out by the operating steps of first providing a crystalline silicon face plate that has a multiplicity of cavities formed in a front surface, and then bonding a backing plate to a back surface of the face plate.
The method may further include the step of forming a multiplicity of cavities in the front surface of the crystalline silicon plate along a crystallographic orientation of  less than 100 greater than . Each of the multiplicity of cavities may be formed in a pyramidal shape. The method may further include the step of transferring a multiplicity of solder balls to an electronic substrate in a molten solder screening process. The method may further include the step of forming the backing plate with a material which has a rigidity or coefficient of thermal expansion substantially similar to that of the crystalline silicon.
The method for preparing a hybrid mold may further include the step of forming the backing plate with a ceramic or a glass, bonding the backing plate to a back surface of a face plate by adhesive means, or bonding the backing plate to the back surface of the face plate by a thermal ionic means. The method may further include the step of forming the backing plate by casting a molten glass or a flowable polymer around the face plate. The method may further include the step of bonding the backing plate to the face plate by a polymeric based adhesive.
The present invention is further directed to a method for preparing a hybrid mold for a multiplicity of solder balls by the operating steps of first providing a crystalline silicon face plate which has a substantially parallel front surface and back surface, then etching a multiplicity of cavities for forming solder balls in the front surface of the face plate, then providing a backing plate which has a top surface and a bottom surface, the top surface has a recess formed therein, and bonding the back surface of the face plate into the recess such that the front surface of the face plate and the top surface of the backing plate are substantially coplanar.
The method for preparing a hybrid mold may further include the step of forming the multiplicity of cavities in the front surface of the face plate along a preselected crystallographic orientation of  less than 100 greater than . The method may further include the step of etching each of the multiplicity of cavities in a pyramidal shape, providing the recess in the top surface of the backing plate with an area and a depth sufficient for receiving the face plate, and forming the backing plate with a material that has a rigidity and coefficient of thermal expansion substantially similar to those of crystalline silicon. The method may further include the step of forming the backing plate with borosilicate glass.
The present invention is still further directed to a method for preparing a hybrid mold for a multiplicity of solder balls by the steps of first providing a crystalline silicon face plate that has a first diameter and a multiplicity of cavities formed in a front surface, then providing a backing plate which has a top surface bonded to a back surface of the face plate, the backing plate has a length and a width larger than the first diameter of the face plate, then providing a frame member which has substantially the same length and width as the backing plate and an aperture with a second diameter larger than the first diameter of the face plate, and bonding an outer peripheral surface of the face plate to an inner peripheral surface of the frame member by an adhesive means such that a top surface of the frame member is coplanar with the front surface of the face plate.
The method for preparing a hybrid mold may further include the step of adhesively bonding a bottom surface of the frame member to a top surface of the backing plate, forming the frame member and the backing plate with substantially the same ceramic material, and forming the frame member and the backing plate with a material that has a rigidity and coefficient of thermal expansion substantially similar to those of crystalline silicon.
The method for preparing a hybrid mold may further include the step of forming the frame member and the backing plate with a material that has a coefficient of thermal expansion within 50% of that for the crystalline silicon. The method may further include the step of bonding the top surface of the backing plate and the back surface of the face plate by a thermal ionic bonding method, etching a multiplicity of cavities in the front surface of the crystalline silicon face plate along a crystallographic orientation of  less than 100 greater than , and etching each of the multiplicity of cavities in the front surface of the crystalline silicon face plate in a pyramidal shape.
The present invention is still further directed to a method for preparing a hybrid mold for a multiplicity of solder balls which can be carried out by the operating steps of first providing a crystalline silicon face plate with a front surface and a back surface, etching a multiplicity of cavities for the solder balls in the front surface, and encasing the face plate with a liquid material forming a backing plate wherein the front surface of the face plate is exposed and the top surface of the backing plate is substantially coplanar with the front surface of the face plate. The method may further include the step of forming the backing plate with a material that has a rigidity and a coefficient of thermal expansion after solidifying that are substantially similar to those for the crystalline silicon. The method may further include the step of etching each of the multiplicity of cavities in the face plate along a crystallographic orientation of  less than 100 greater than . The method may further include the step of forming the backing plate with borosilicate glass or a polymeric material.
The present invention is still further directed to a method for molding a multiplicity of solder balls in a hybrid mold which can be carried out by the operating steps of first providing a crystalline silicon face plate that has a front surface and a back surface substantially parallel to each other, etching a multiplicity of cavities in the front surface of the face plate, bonding a backing plate to the back surface of the face surface, and then filling the multiplicity of cavities with a molten solder. The method may further include the step of etching the multiplicity of cavities in the crystalline silicon face plate along a crystallographic orientation of  less than 100 greater than .